1. Field of the Invention
The present invention relates to an organic compound that can be used as a light-emitting element material. The present invention relates to a light-emitting element, a display module, a lighting module, a light-emitting device, a display device, a lighting device, and an electronic device each using the organic compound.
2. Description of the Related Art
As next generation lighting devices or display devices, display devices using light-emitting elements (organic EL elements) in which organic compounds are used as light-emitting substances have been rapidly developed because of their potential for thinness, lightness, high speed response to input signals, low power consumption, and the like.
In an organic EL element, voltage application between electrodes, between which a light-emitting layer is interposed, causes recombination of electrons and holes injected from the electrodes, which brings a light-emitting substance into an excited state, and the return from the excited state to the ground state is accompanied by light emission. Since the wavelength of light emitted from a light-emitting substance depends on the light-emitting substance, use of different types of organic compounds as light-emitting substances makes it possible to obtain light-emitting elements which exhibit various wavelengths, i.e., various colors.
In the case of display devices which are used to display images, such as displays, at least three-color light, i.e., red light, green light, and blue light is necessary for reproduction of full-color images. Furthermore, in application to lighting devices, it is ideal to obtain light having wavelength components evenly spreading in the visible light region for obtaining a high color rendering property, but in reality, light obtained by mixing two or more kinds of light having different wavelengths is used for lighting application in many cases. It is known that, with a mixture of three-color light, i.e., red light, green light, and blue light, white light having a high color rendering property can be obtained.
Light emitted from a light-emitting substance is peculiar to the substance as described above. However, important performances as a light-emitting element, such as a lifetime, power consumption, and even emission efficiency, are not only dependent on the light-emitting substance but also greatly dependent on layers other than the light-emitting layer, an element structure, properties of an emission center substance and a host material, compatibility between them, carrier balance, and the like. Therefore, there is no doubt that many kinds of light-emitting element materials are necessary for a growth in this field. For the above-described reasons, light-emitting element materials with a variety of molecular structures have been suggested (e.g., see Patent Document 1).
As is generally known, the generation ratio of a singlet excited state to a triplet excited state in a light-emitting element using electroluminescence is 1:3. Therefore, a light-emitting element in which a phosphorescent material capable of converting the triplet excited state to light emission is used as an emission center substance can theoretically obtain higher emission efficiency than a light-emitting element in which a fluorescent material capable of converting the singlet excited state to light emission is used as an emission center substance.
As a host material in a host-guest type light-emitting layer or a substance contained in each transport layer in contact with a light-emitting layer, a substance having a wider band gap or a higher triplet level (T1, a larger energy difference between a triplet excited state and a singlet ground state) than an emission center substance is used for efficient conversion of excitation energy into light emission from the emission center substance.
However, most substances that are used as a host material of the light-emitting element are fluorescent materials, in which electron transition between different states is forbidden. The triplet excited state of the material is at a lower energy level than the singlet excited state of the material, which means that a host material for obtaining phosphorescence needs to have a wider band gap than a host material for obtaining fluorescence of the same wavelength.
Therefore, a host material and a carrier-transport material each having a further wider band gap are necessary in order to efficiently obtain phosphorescence. However, it is extremely difficult to develop a substance to be a light-emitting element material which has such a wide band gap while enabling a balance between important characteristics of a light-emitting element, such as low driving voltage and high emission efficiency.